A conventional refrigeration system includes a compressor that compresses refrigerant vapor. The refrigerant vapor from the compressor is directed into a condenser coil where the vapor is liquefied at high pressure. The high pressure liquid refrigerant is then generally delivered to a receiver tank. The high pressure liquid refrigerant from the receiver tank flows from the receiver tank to an evaporator coil after it is expanded by an expansion valve to a low pressure two-phase refrigerant. As the low pressure two-phase refrigerant flows through the evaporator coil, the refrigerant absorbs heat from the refrigeration case and boils off to a single phase low pressure vapor that finally returns to the compressor where the closed loop refrigeration process repeats itself.
In some systems, the refrigeration system will include multiple compressors connected to multiple circuits where a circuit is defined as a physically plumbed series of cases operating at the same pressure/temperature. For example, in a grocery store, one set of cases within a circuit may be used for frozen food, another set used for meats, while another set is used for dairy. Each circuit having a group of cases will thus operate at different temperatures. These differences in temperature are generally achieved by using mechanical evaporator pressure regulators (EPR) or valves located in series with each circuit. Each mechanical evaporator pressure regulator regulates the pressure for all the cases connected within a given circuit. The pressure at which the evaporator pressure regulator controls the circuit is adjusted once during the system start-up using a mechanical pilot screw adjustment present in the valve. The pressure regulation point is selected based on case temperature requirements and pressure drop between the cases and the rack suction pressure.
The multiple compressors are also piped together using suction and discharge gas headers to form a compressor rack consisting of the multiple compressors in parallel. The suction pressure for the compressor rack is controlled by modulating each of the compressors on and off in a controlled fashion. The suction pressure set point for the rack is generally set to a value that can meet the lowest evaporator circuit requirement. In other words, the circuit that operates at the lowest temperature generally controls the suction pressure set point which is fixed to support this circuit.
There are, however, various disadvantages of running and controlling a system in this manner. For example, one disadvantage is that the requirement for the case temperature generally changes throughout the year. This requires a refrigeration mechanic to perform an in-situ change of evaporator pressure settings, via the pilot screw adjustment of each evaporator pressure regulator, thereby further requiring re-adjustment of the fixed suction pressure set point at the rack of compressors. Another disadvantage of this type of control system is that case loads change from winter to summer. Thus, in the winter, there is a lower case load which requires a higher suction pressure set point and in the summer there is a higher load requiring a lower suction pressure set point. However, in the real world, such adjustments are seldom done since they also require manual adjustment by way of a refrigeration mechanic.
What is needed then is a method and apparatus for refrigeration system control which utilizes electronic evaporator pressure regulators and a floating suction pressure set point for the rack of compressors which does not suffer from the above mentioned disadvantages. This, in turn, will provide adaptive adjustment of the evaporator pressure for each circuit, adaptive adjustment of the rack suction pressure, enable changing evaporator pressure requirements remotely, enable adaptive changes in pressure settings for each circuit throughout its operation so that the rack suction pressure is operated at its highest possible value, enable floating circuit temperature based on a product simulator probe, and enable the use of case temperature information to control the evaporator pressure for the whole circuit and the suction pressure at the compressor rack. It is, therefore, an object of the present invention to provide such a method and apparatus for refrigeration system control using electronic evaporator pressure regulators and a floating suction pressure set point.